1. Field of the Invention
This invention pertains to the field of image display devices, and more particularly to liquid crystal display devices, and to calibration circuitry for such devices.
2. Description of the Related Art
Image display devices such as liquid crystal display (LCD) devices are widely known. With reference to the following description, familiarity with conventional features of such devices will be assumed, so that only features bearing on the present invention will be described.
FIG. 1 shows relevant portions of an exemplary liquid crystal display (LCD) device 100.
The LCD device 100 comprises in relevant part: a plurality of pixels 110; a plurality of column (data) lines 120 connected to the plurality of pixels 110; a plurality of column (data) drivers 130 for supplying data to pixels 110 via the column lines 120; a plurality of column driver switches 140; a plurality of row (scanning) lines 150 connected to rows of pixels 110; and a plurality of row drivers 160 connected to the row lines 120 for selecting a row of pixels 110 to which data from the column drivers 130 is to be applied.
Typically, each pixel 110 includes a pixel switching device 112 and a storage device (pixel capacitor) 114. The pixel switching device 112, which may be a thin film transistor (TFT), is responsive to a scanning signal on the connected row line 150 to switch a data signal applied via the connected column line 120 into the storage device 114.
The LCD device 100 may be a liquid crystal on silicon (LCOS) type LCD device. In that case, the column (data) drivers 130, column driver switches 140, and/or row (scanning) drivers 160 may be integrated onto a same silicon substrate as the liquid crystal pixels 110.
Image data is provided as digital input data from an external video generator to the column drivers 130. However, the column drivers 130 must provide analog image data to the column lines 120. Hence, the image data is subjected to signal processing, including digital to analog conversion, in the column drivers 130.
Some problems with the prior art LCD device 100 will now be explained.
Variations between the column drivers 130 and column lines 120 cause a situation wherein the pixels 110 of two different column lines 120 may display different brightnesses (intensities) even though the same digital image data is applied to the column driver(s) 130 for both column lines 120. Indeed, the variations may be so great that a situation occurs wherein a column driver 130 for a first column line 120 receives first digital image data having a greater value than second digital image data received by a column driver 130 for a second column line 120, and yet the pixels 110 of the second column line 120 actually display a brighter image (greater intensity) than the pixels 110 of the first column line 120. These variations result in an undesirable display characteristic.
Moreover, the signal processing in the column drivers 130 produces non-linearities in the image data. Because of these non-linearities, the brightness range of the image data does not monotonically increase. In other words, one or more situations may occur wherein the digital image data value for a particular column line 120 is increased, but the actual displayed brightness displayed by the pixels 110 of the column line 120 decreases.
In general, propagation delays of digital and analog signals in the device 100, in addition to common circuit property variations (e.g., amplifier offsets; gain/bandwidth variations) cause brightness variations between pixels or regions (e.g., columns) of the display.
Accordingly, it would be desirable to provide an image display device with reduced or eliminated brightness level variations among pixels or columns receiving the same digital input data. It also would be desirable to provide an image display device having a brightness that monotonically increases in response to digital input data received form an external video signal generator.
Accordingly, in one aspect, an image display device includes a plurality of pixels arranged in a matrix or rows and columns, a plurality of column lines each connected to a corresponding one of the columns of pixels, at least one column driver providing a data voltage to one of the column lines, a generator producing a reference voltage, and means for comparing the reference voltage to the data voltage and in response thereto producing a calibration data error value.
In another aspect, a method of calibrating data voltage levels for image display device including a plurality of pixels arranged in a matrix of rows and columns, a plurality of column lines connected to the plurality of pixels, and a plurality of column drivers connected to the column lines and providing data to the pixels, includes: generating a reference signal; receiving P-bit digital input data having a digital input data value; producing a data voltage on one of the column lines in response to the received digital input data; and comparing the reference signal to the data voltage produced on one of the column lines and, in response thereto, generating a calibration data error value.